Devin Alex

While internal quality control measures and rigorous adherence to standards like ISO/IEC 17025 are essential, a laboratory's ultimate proof of competence is its ability to generate accurate and consistent results when compared against its peers. This external validation is achieved through Interlaboratory Comparisons (ILC) and Proficiency Testing (PT).

Proficiency Testing is formally defined as the evaluation of participant performance against pre-established criteria by means of interlaboratory comparisons. It is an empirical demonstration that a laboratory's people, equipment, environment, and procedures are working together correctly.

The Mechanics of a PT Scheme

PT programs are typically organized by specialized Proficiency Testing Providers (PTPs) who operate in accordance with ISO/IEC 17043. The process generally follows these steps:

Design and Logistics: The PTP designs a scheme relevant to a specific scope of calibration or testing. They prepare a stable, homogeneous artifact (the PT item).
Circulation: The PT item is circulated sequentially (or simultaneously, if multiple identical items exist) to participating laboratories.
Measurement: Participating laboratories perform measurements on the item using their standard, accredited procedures, unaware of the assigned reference value. They report their measurement result and their claimed expanded measurement uncertainty back to the PTP.
Reference Value Determination: The PTP determines the assigned value for the artifact. In metrology, this is often done by having a highly competent Reference Laboratory (often a National Metrology Institute) characterize the artifact before, and sometimes after, circulation to ensure stability.
Performance Evaluation: The PTP uses statistical methods to evaluate the performance of each participant against the assigned value.

Statistical Evaluation: The $E_n$ Number

In calibration PT schemes, the primary statistical tool used to evaluate performance is the normalized error ratio, or $E_n$ number. Unlike a simple z-score used in analytical chemistry, the $E_n$ number explicitly accounts for the measurement uncertainty claimed by both the participant and the reference laboratory.

$E_n = \frac{x - X}{\sqrt{U_{lab}^2 + U_{ref}^2}}$

Where:

$x$ is the participant's measured value.
$X$ is the assigned reference value.
$U_{lab}$ is the participant's reported expanded uncertainty.
$U_{ref}$ is the reference laboratory's expanded uncertainty.

The interpretation of the $E_n$ number is straightforward:

$|E_n| \le 1.0$ : Satisfactory performance. The participant's result agrees with the reference value within their claimed uncertainties.
$|E_n| > 1.0$ : Unsatisfactory performance. The difference between the participant's result and the reference value is too large to be explained by the stated uncertainties.

Addressing Unsatisfactory Results

An $|E_n| > 1.0$ is a critical event. It indicates a failure in the measurement process and requires immediate action per ISO/IEC 17025. The laboratory must initiate a formal corrective action process to identify the root cause of the failure.

Common root causes for PT failures include:

Underestimated Uncertainty: The laboratory failed to identify or properly quantify a significant source of error in their uncertainty budget.
Equipment Malfunction: An undetected drift, damage, or out-of-tolerance condition in the reference standards or working instruments.
Operator Error: Misinterpretation of the procedure, incorrect setup, or simple transcription errors.
Environmental Influences: Uncontrolled temperature, humidity, or vibration affecting the measurement.

A robust root cause analysis must be followed by corrective action, and the laboratory typically must participate in another PT round to demonstrate that the issue has been effectively resolved.